The present invention relates to a process for preparing ethylene copolymers in the presence of free-radical polymerization initiator at pressures in the range of from 160 MPa to 350 MPa and temperatures in the range of from 100° C. to 350° C. in a tubular reactor by copolymerizing ethylene, a bi- or multifunctional comonomer and optionally further comonomers and it further relates to ethylene copolymers obtainable by such a process, to the use of the ethylene copolymers for extrusion coating and to a process for extrusion coating a substrate selected from the group consisting of paper, paperboard, polymeric film, and metal, with such ethylene copolymers.
Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE). LDPE is a versatile polymer which can be used in a variety of applications, such as film, coating, molding, and wire and cable insulation. There is consequently still demand for optimizing the processes for its preparation.
Common reactors for preparing LDPE polymers at high pressures are either tubular reactors or stirred autoclave reactors. The advantages of polymerizing in a tubular reactor are that higher turnovers can be achieved in the polymerization process, the process is easier to scale-up and it is accordingly possible to build “world-scale” plants and the polymerization is in general more economic because of a lower specific consumption of utilities such as electricity and cooling water. However, the LDPE polymers prepared in a tubular high-pressure reactor have certain disadvantages for some applications. Compared to LDPE polymers of similar melt flow rate (MFR) and density prepared in a high-pressure autoclave LDPE reactor the LDPE polymers prepared in a tubular reactor have in general a narrower molecular weight distribution and a lower amount of long-chain branching (LCB).
An example for an application, in which LDPE prepared in a tubular reactor is inferior to LDPE prepared in an autoclave reactor, is extrusion coating. In this process, the molten LDPE is extruded through a slit-die and casted into a film, which is then coated onto a substrate such as paper, paperboard, a polymeric film like a polyethylenterephthalat (PET) film or a biaxially-oriented polypropylene (BOPP) film, or a metal like an aluminum foil. For a good processability, the LDPE has to show a stable web, i.e. the film casted out of the die shall not oscillate, and a low neck-in is required, i.e. the ratio of the width of the film over the width of the die should not be too low. Furthermore, high processing temperatures of up to 350° C. are required for the post-treatment of the produced polymer film in order to enhance its adhesion properties at substrates such as metal, paper, or paperboard. To fulfill these requirements a certain level of LCB is advantageous.
For providing a LDPE copolymer that enhances the web stability during the extrusion coating process, WO 2006/094723 discloses a process for the preparation of an ethylene copolymer in a tubular reactor, in which ethylene is copolymerized with a di- or higher functional (meth)acrylate. WO 2007/110127 teaches to copolymerize ethylene and a bifunctional α,ω-alkadiene for the same purpose. However, these bi- or higher functional comonomers can only be used in relatively small amounts because otherwise a too high amount of very high-molecular component is produced, corresponding to a high risk of gel formation, especially if these comonomers are not perfectly homogenized in the reaction medium. Furthermore, the tendency for forming LCB is limited.